WASHINGTON -- Former Trump campaign chairman Paul Manafort agreed Friday to co-operate with the special counsel's Russia investigation as he pleaded guilty to federal charges and avoided a second trial that could have exposed him to even greater punishment.

The deal gives special counsel Robert Mueller a key co-operator who led the Trump election effort for a crucial stretch during the 2016 presidential campaign. The result also ensures the investigation will extend far beyond the November congressional elections despite entreaties from the president's lawyers that Mueller bring his probe to a close.

It is unclear what information Manafort is prepared to provide to investigators about President Donald Trump or that could aid Mueller's investigation into Russian interference in the 2016 presidential election. But the plea nonetheless makes Manafort the latest associate of Trump, a president known to place a premium on loyalty among subordinates, to admit guilt and co-operate with investigators in hopes of leniency.

In the past year, Mueller has secured pleas from a former national security adviser who lied to the FBI about discussing sanctions with a Russian ambassador, a campaign aide who broached the idea of a meeting with Russian President Vladimir Putin; and another aide who was indicted alongside Manafort but ultimately turned on him. The president's former personal lawyer has separately pleaded guilty in New York.

Friday's deal, to charges tied to Ukrainian political consulting work but unrelated to the campaign, was struck just days before Manafort was to have stood trial for a second time.

He was convicted last month of eight financial crimes in a separate trial in Virginia and faces seven to 10 years in prison in that case. The two conspiracy counts he pleaded guilty to on Friday carry up to five years in prison, though Manafort's sentence will ultimately depend on his co-operation.

He smiled broadly as he entered the courtroom Friday but gave terse and barely audible answers during questioning from the judge.

"He wanted to make sure that his family was able to remain safe and live a good life. He's accepted responsibility. This is for conduct that dates back many years and everybody should remember that," said attorney Kevin Downing.

The co-operation deal requires Manafort to provide whatever information the government asks of him, though it does not specify what if anything prosecutors hope to receive about Trump.

Given his direct involvement in the Trump campaign, including episodes being scrutinized by Mueller, Manafort could be positioned to provide key insight for investigators working to establish whether the campaign co-ordinated with Russia.

Manafort was among the participants, for instance, in a June 2016 Trump Tower meeting with Russians and the president's oldest son and son-in-law that was arranged so the campaign could receive derogatory information about Democrat Hillary Clinton. A grand jury used by Mueller has heard testimony about the meeting.

He was also a close business associate of a man who U.S. intelligence believes has ties to Russian intelligence. And while he was working on the Trump campaign, emails show Manafort discussed providing private briefings for a wealthy Russian businessman close to Vladimir Putin.

White House press secretary Sarah Sanders insisted that the Manafort case was unrelated to Trump.

"This had absolutely nothing to do with the president or his victorious 2016 presidential campaign. It is totally unrelated."

Added Trump lawyer Rudy Giuliani: "Once again an investigation has concluded with a plea having nothing to do with President Trump or the Trump campaign."

Under the terms of the deal, Manafort was allowed to plead guilty to just two conspiracy counts, though the crimes he admitted cover the same conduct alleged in an indictment last year. Manafort's homes in New York City, in the Hamptons and in Virginia, as well as money from his bank accounts and life insurance policies may be seized by the government as part of the deal.

It's unclear how the deal might affect any Manafort pursuit of a pardon from Trump. The president has signalled that he's sympathetic to Manafort's cause. In comments to Politico before the plea deal, Giuliani said a plea without a co-operation agreement wouldn't foreclose the possibility of a pardon.

Manafort had aggressively fought the charges against him and taken shots at his co-defendant, Rick Gates, who cut a deal with prosecutors earlier this year that included a co-operation agreement.

At the time of Gates' plea, Manafort issued a statement saying he "had hoped and expected my business colleague would have had the strength to continue the battle to prove our innocence." And during his Virginia trial in August, Manafort's lawyers spent considerable time painting Gates as a liar, embezzler, philanderer and turncoat who would say anything to get a lighter prison sentence.

Pleading guilty allows Manafort to avoid a trial that was expected to last at least three weeks and posed the potential of adding years to the time he is already facing under federal sentencing guidelines from his conviction in Virginia.

A jury in that earlier trial found Manafort guilty of eight counts of tax evasion, failing to report foreign bank accounts and bank fraud. Jurors deadlocked on 10 other counts.

In the current Washington case, prosecutors had been expected to lay out in detail Manafort's political consulting and lobbying work on behalf of former Ukrainian President Viktor Yanukovych and the pro-Russian Party of Regions.

Prosecutors say that Manafort directed a large scale lobbying operation in the U.S. for Ukrainian interests without registering with the Justice Department as required by the federal Foreign Agents Registration Act. Manafort was accused of concealing from the IRS tens of millions of dollars in proceeds from his Ukrainian patrons and conspiring to launder that money through offshore accounts in Cyprus and elsewhere.

Manafort had denied the allegations and pleaded not guilty. After his indictment last October, prosecutors say he continued to commit crimes by tampering with witnesses.